A honeycomb structure 100 used as a DPF is formed by bonding and integrating a plurality of honeycomb segments 200 having the same shape and size by a bonding material 900, forming into a predetermined shape such as a circular cross section, and covering the exterior with a coating layer 400, as shown in FIG. 1A. This honeycomb structure 100 is located in an exhaust system of a diesel engine and used for purifying exhaust gas.
As shown in FIGS. 1B and 1C, each of the honeycomb segments 200 is made of a silicon carbide porous material having a plurality of through holes 500 separated from each other by porous partition walls 600. The through holes 500 pass through the honeycomb segment 200 along a one axis. One side ends of the through holes 500 are sealed alternately with a sealing material 7. On the other hand, the other side end of the through holes 500 not sealed is sealed. In other words, a certain through hole 500 has a left end open while a right end is sealed with the sealing material 7. Other through hole 5 adjacent to the certain through hole 500 has the left end sealed with the sealing material 7 and the right end open. With such a structure, as indicated by arrows in FIG. 1C, exhaust gas entered into the through hole 500 having the left side open passes through the porous partition walls 600 and flows out from the other through holes 500. In addition, the exhaust gas may be purified since the partition walls 600 trap particulates in the exhaust gas when the gas passes through the partition walls 600.
As the bonding material 900 bonding the honeycomb segments 200, a material is available, adding an inorganic fiber such as ceramic fiber, an organic or inorganic binder, and a dispersion medium such as water to a ceramic powder equivalent to component substances of the honeycomb segments 200. Generally, in order to suppress temperature rise in the honeycomb segments 2 when regenerating the honeycomb structure 100, the bonding material 900 having a larger heat capacity than the honeycomb segments 2 is used.
Using the honeycomb structure 100 continuously, soot deposits on the partition walls 600 and pressure loss increases over time. Such increase of the pressure loss causes degradation in performance of the engine. Therefore, regeneration of the honeycomb structure 100 is performed by burning and removing deposited soot.
The regeneration is performed by heating the honeycomb structure 1 to approximately 550 to 600 degrees Centigrade while the automobile is moving. Through this heating, the soot burns and heats itself, thereby raising the temperature of the entire honeycomb structure 100. Due to this temperature rise, an excessive temperature gradient occurs in the central part of the honeycomb segments, particularly near the center of a cross section of the honeycomb structure 100, the cross section is perpendicular to the through hole length (along the axis). Therefore, it arises a state that cracks caused by thermal stress are easily generated.
To solve these problems, in the prior art described above, the bonding material 900 bonding the honeycomb segments 200, which has a large heat capacity, is used for suppressing the temperature rise in the honeycomb segments 200 during regeneration and suppressing generation of cracks. In addition, a configuration is under review, in which layers of the bonding material 900 having a large heat capacity are located in the central part of the honeycomb structure 100 where the cracks are easily generated, and the temperature gradient in the central part is suppressed.
Amount of soot that allows regeneration of the honeycomb structure without generating cracks is called ‘maximum soot amount for regeneration’ of the honeycomb structure. It is preferable that the ‘maximum soot amount for regeneration’ is greater, since frequency of regeneration may be decreased.
The honeycomb structure in which the bonding material 900 is located in the center of the honeycomb structure, that is, the honeycomb structure in which the honeycomb segments 200 are located so that the layers of the bonding material 900 are crossing in the center of honeycomb structure 100, may suppress the generation of the cracks effectively and increase ‘maximum soot amount for regeneration’, compared to the honeycomb structure in which the honeycomb segments 200 are located so that the through holes are located in the center, that is, the honeycomb structure in which the layer of bonding material 900 is located in the part shifted from the center. Therefore, it is possible to perform the regeneration in the state that more soot is deposited, by locating the layers of the bonding material 900 in the central part of the honeycomb structure 100.
However, according to recent specific honeycomb structures for automobile, due to structural restriction in the automobile at installation locations, a shape of the section perpendicular to an axis is not limited to a symmetrical shape such as a circle (the honeycomb structure shown in FIG. 1A) or a square, and structures having irregular shape of cross sections, difficult to determine the center, are increasing.
In a honeycomb structure having such irregular shape of cross section, it is not easy to adjust the position of the honeycomb segments so that the layers of bonding material 900 are arranged in the center, since it is difficult to determine the center.
Furthermore, increasing the amount of the bonding material 900 in the honeycomb structure by increasing the thicknesses of the layer of the bonding material causes increase in heat capacity, thereby suppressing temperature rise and temperature gradient in the central part during the regeneration. However, increasing the amount of the bonding material 900 causes increase in the cross sectional areas of the layers of the bonding material 900, resulting in relative decrease in the total cross sectional areas of the honeycomb segments. Accordingly, the total volume of the through holes reduces, and capacity of removing the soot reduces. Therefore, simply increasing the amount of the bonding material 900 does not effectively increase the ‘maximum soot amount for regeneration’.